Network Working Group C. Huitema
Internet-Draft Private Octopus Inc.
Intended status: Informational June 29, 2018
Expires: December 31, 2018
DNS-SD Privacy and Security Requirementsdraft-huitema-dnssd-prireq-00
Abstract
DNS-SD (DNS Service Discovery) normally discloses information about
devices offering and requesting services. This information includes
host names, network parameters, and possibly a further description of
the corresponding service instance. Especially when mobile devices
engage in DNS Service Discovery over Multicast DNS at a public
hotspot, serious privacy problems arise. We analyze the requirements
of a privacy respecting discovery service.
Status of This Memo
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This Internet-Draft will expire on December 31, 2018.
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the Wi-Fi network of an Internet cafe, or two travelers who want to
share files between their laptops when waiting for their plane in an
airport lounge.
We expect that these exchanges will start with a discovery procedure
using DNS-SD [RFC6763] over mDNS [RFC6762]. One of the devices will
publish the availability of a service, such as a picture library or a
file store in our examples. The user of the other device will
discover this service, and then connect to it.
When analyzing these scenarios in Section 3, we find that the DNS-SD
messages leak identifying information such as the instance name, the
host name or service properties.
1.1. Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. DNS-SD Discovery Scenarios
DNS-Based Service Discovery (DNS-SD) is defined in [RFC6763]. It
allows nodes to publish the availability of an instance of a service
by inserting specific records in the DNS ([RFC1033], [RFC1034],
[RFC1035]) or by publishing these records locally using multicast DNS
(mDNS) [RFC6762]. Available services are described using three types
of records:
PTR Record: Associates a service type in the domain with an
"instance" name of this service type.
SRV Record: Provides the node name, port number, priority and weight
associated with the service instance, in conformance with
[RFC2782].
TXT Record: Provides a set of attribute-value pairs describing
specific properties of the service instance.
In the remaining sections, we review common discovery scenarios
provided by DNS-SD and discuss their privacy requirements.
2.1. Private client and public server
Perhaps the simplest private discovery scenario involves a single
client connecting to a public server through a public network. A
common example would be a traveler using a publicly available printer
in a business center, in an hotel or at an airport.
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( Taking notes:
( David is printing
( a document
~~~~~~~~~~~
o
___ o ___
/ \ _|___|_
| | |* *|
\_/ __ \_/
| / / Discovery +----------+ |
/|\ /_/ <-----------> | +----+ | /|\
/ | \__/ +--| |--+ / | \
/ | |____/ / | \
/ | / | \
/ \ / \
/ \ / \
/ \ / \
/ \ / \
/ \ / \
In that scenario, the server is public and wants to be discovered,
but the client is private. The adversary will be listening to the
network traffic, trying to identify the visitors' devices and their
activity. Identifying devices leads to identifying people, either
just for tracking people or as a preliminary to targeted attacks.
The requirement in that scenario is that the discovery activity
should not disclose the identity of the client.
2.2. Private client and private server
The second private discovery scenario involves private client
connecting to a private server. A common example would be two people
engaging in a collaborative application in a public place, such as
for example an airport's lounge.
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( Taking notes:
( David is meeting
( with Stuart
~~~~~~~~~~~
o
___ ___ o ___
/ \ / \ _|___|_
| | | | |* *|
\_/ __ __ \_/ \_/
| / / Discovery \ \ | |
/|\ /_/ <-----------> \_\ /|\ /|\
/ | \__/ \__/ | \ / | \
/ | | \ / | \
/ | | \ / | \
/ \ / \ / \
/ \ / \ / \
/ \ / \ / \
/ \ / \ / \
/ \ / \ / \
In that scenario, the collaborative application on one of the device
will act as server, and the application on the other device will act
as client. The server wants to be discovered by the client, but has
no desire to be discovered by anyone else. The adversary will be
listening to network traffic, attempting to discover the identity of
devices as in the first scenario, and also attempting to discover the
patterns of traffic, as these patterns reveal the business and social
interactions between the owners of the devices.
The requirement in that scenario is that the discovery activity
should not disclose the identity of either the client or the server.
2.3. Wearable client and server
The third private discovery scenario involves wearable devices. A
typical example would be the watch on someone's wrist connecting to
the phone in their pocket.
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( Taking notes:
( David' is here. His watch is
( talking to his phone
~~~~~~~~~~~
o
___ o ___
/ \ _|___|_
| | |* *|
\_/ \_/
| _/ |
/|\ // /|\
/ | \__/ ^ / | \
/ |__ | Discovery / | \
/ |\ \ v / | \
/ \\_\ / \
/ \ / \
/ \ / \
/ \ / \
/ \ / \
This third scenario is in many ways similar to the second scenario.
It involves two devices, one acting as server and the other acting as
client, and it leads to the same requirement that the discovery
traffic not disclose the identity of either the client or the server.
The main difference is that the devices are managed by a single
owner, which can lead to different methods for establishing secure
relations between the device. There is also an added emphasis in
hiding the type of devices that the person wears.
In addition to tracking the identity of the owner of the devices, the
adversary is interested by the characteristics of the devices, such
as type, brand, and model. Identifying the type of device can lead
to further attacks, from theft to device specific hacking. The
combination of devices worn by the same person will also provide a
"fingerprint" of the person, allowing identification.
3. Privacy Considerations
The discovery scenarios in Section Section 2 illustrate three
separate privacy requirements that vary based on use case:
1. Client identity privacy: Client identities are not leaked during
service discovery or use.
2. Multi-owner, mutual client and server identity privacy: Neither
client nor server identities are leaked during service discovery
or use.
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3. Single-owner, mutual client and server identity privacy:
Identities of clients and servers owned and managed by the same
application, device, or user are not leaked during service
discovery or use.
In the remaining subsections, we describe aspects of DNS-SD that make
these requirements difficult to achieve in practice.
3.1. Privacy Implication of Publishing Service Instance Names
In the first phase of discovery, client obtain all PTR records
associated with a service type in a given naming domain. Each PTR
record contains a Service Instance Name defined in Section 4 of
[RFC6763]:
Service Instance Name = <Instance> . <Service> . <Domain>
The <Instance> portion of the Service Instance Name is meant to
convey enough information for users of discovery clients to easily
select the desired service instance. Nodes that use DNS-SD over mDNS
[RFC6762] in a mobile environment will rely on the specificity of the
instance name to identify the desired service instance. In our
example of users wanting to upload pictures to a laptop in an
Internet Cafe, the list of available service instances may look like:
Alice's Images . _imageStore._tcp . local
Alice's Mobile Phone . _presence._tcp . local
Alice's Notebook . _presence._tcp . local
Bob's Notebook . _presence._tcp . local
Carol's Notebook . _presence._tcp . local
Alice will see the list on her phone and understand intuitively that
she should pick the first item. The discovery will "just work".
However, DNS-SD/mDNS will reveal to anybody that Alice is currently
visiting the Internet Cafe. It further discloses the fact that she
uses two devices, shares an image store, and uses a chat application
supporting the _presence protocol on both of her devices. She might
currently chat with Bob or Carol, as they are also using a _presence
supporting chat application. This information is not just available
to devices actively browsing for and offering services, but to
anybody passively listening to the network traffic.
3.2. Privacy Implication of Publishing Node Names
The SRV records contain the DNS name of the node publishing the
service. Typical implementations construct this DNS name by
concatenating the "host name" of the node with the name of the local
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domain. The privacy implications of this practice are reviewed in
[RFC8117]. Depending on naming practices, the host name is either a
strong identifier of the device, or at a minimum a partial
identifier. It enables tracking of both the device, and, by
extension, the device's owner.
3.3. Privacy Implication of Publishing Service Attributes
The TXT record's attribute-value pairs contain information on the
characteristics of the corresponding service instance. This in turn
reveals information about the devices that publish services. The
amount of information varies widely with the particular service and
its implementation:
o Some attributes like the paper size available in a printer, are
the same on many devices, and thus only provide limited
information to a tracker.
o Attributes that have freeform values, such as the name of a
directory, may reveal much more information.
Combinations of attributes have more information power than specific
attributes, and can potentially be used for "fingerprinting" a
specific device.
Information contained in TXT records does not only breach privacy by
making devices trackable, but might directly contain private
information about the user. For instance the _presence service
reveals the "chat status" to everyone in the same network. Users
might not be aware of that.
Further, TXT records often contain version information about services
allowing potential attackers to identify devices running exploit-
prone versions of a certain service.
3.4. Device Fingerprinting
The combination of information published in DNS-SD has the potential
to provide a "fingerprint" of a specific device. Such information
includes:
o List of services published by the device, which can be retrieved
because the SRV records will point to the same host name.
o Specific attributes describing these services.
o Port numbers used by the services.
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o Priority and weight attributes in the SRV records.
This combination of services and attributes will often be sufficient
to identify the version of the software running on a device. If a
device publishes many services with rich sets of attributes, the
combination may be sufficient to identify the specific device.
A sometimes heard argument is that devices providing services can be
identified by observing the local traffic, and that trying to hide
the presence of the service is futile. This argument, however, does
not carry much weight because
1. Proving privacy at the discovery layer is of the essence for
enabling automatically configured privacy-preserving network
applications. Application layer protocols are not forced to
leverage the offered privacy, but if device tracking is not
prevented at the deeper layers, including the service discovery
layer, obfuscating a certain service's protocol at the
application layer is futile.
2. Further, even if the application layer does not protect privacy,
it is hard to record and analyse the unicast traffic (which most
applications will generate) compared to just listening to the
multicast messages sent by DNS-SD/mDNS.
The same argument can be extended to say that the pattern of services
offered by a device allows for fingerprinting the device. This may
or may not be true, since we can expect that services will be
designed or updated to avoid leaking fingerprints. In any case, the
design of the discovery service should avoid making a bad situation
worse, and should as much as possible avoid providing new
fingerprinting information.
3.5. Privacy Implication of Discovering Services
The consumers of services engage in discovery, and in doing so reveal
some information such as the list of services they are interested in
and the domains in which they are looking for the services. When the
clients select specific instances of services, they reveal their
preference for these instances. This can be benign if the service
type is very common, but it could be more problematic for sensitive
services, such as for example some private messaging services.
One way to protect clients would be to somehow encrypt the requested
service types. Of course, just as we noted in Section 3.4, traffic
analysis can often reveal the service.
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For each of the operations described above, we must also consider
security threats we are concerned about.
4.1. Authenticity, Integrity & Freshness
Can we trust the information we receive? Has it been modified in
flight by an adversary? Do we trust the source of the information?
Is the source of information fresh, i.e., not replayed? Freshness
may or may not be required depending on whether the discovery process
is meant to be online. In some cases, publishing discovery
information to a shared directory or registry, rather than to each
online recipient through a broadcast channel, may suffice.
4.2. Confidentiality
Confidentiality is about restricting information access to only
authorized individuals. Ideally this should only be the appropriate
trusted parties, though it can be challenging to define who are "the
appropriate trusted parties." In some uses cases, this may mean that
only mutually authenticated and trusting clients and servers can read
messages sent for one another. The "Discover" operation in
particular is often used to discover new entities that the device did
not previously know about. It may be tricky to work out how a device
can have an established trust relationship with a new entity it has
never previously communicated with.
4.3. Resistance to Dictionary Attacks
It can be tempting to use (publicly computable) hash functions to
obscure sensitive identifiers. This transforms a sensitive unique
identifier such as an email address into a "scrambled" (but still
unique) identifier. Unfortunately simple solutions may be vulnerable
to offline dictionary attacks.
4.4. Resistance to Denial-of-Service Attack
In any protocol where the receiver of messages has to perform
cryptographic operations on those messages, there is a risk of a
brute-force flooding attack causing the receiver to expend excessive
amounts of CPU time (and battery power) just processing and
discarding those messages.
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Internet-Draft DNS-SD Privacy Requirements June 20184.5. Resistance to Sender Impersonation
Sender impersonation is an attack wherein messages such as service
offers are forged by entities who do not possess the corresponding
secret key material. These attacks may be used to learn the identity
of a communicating party, actively or passively.
4.6. Sender Deniability
Deniability of sender activity, e.g., of broadcasting a discovery
request, may be desirable or necessary in some use cases. This
property ensures that eavesdroppers cannot prove senders issued a
specific message destined for one or more peers.
5. Operational Considerations5.1. Power Management
Many modern devices, especially battery-powered devices, use power
management techniques to conserve energy. One such technique is for
a device to transfer information about itself to a proxy, which will
act on behalf of the device for some functions, while the device
itself goes to sleep to reduce power consumption. When the proxy
determines that some action is required which only the device itself
can perform, the proxy may have some way (such as Ethernet "Magic
Packet") to wake the device.
In many cases, the device may not trust the network proxy
sufficiently to share all its confidential key material with the
proxy. This poses challenges for combining private discovery that
relies on per-query cryptographic operations, with energy-saving
techniques that rely on having (somewhat untrusted) network proxies
answer queries on behalf of sleeping devices.
5.2. Protocol Efficiency
Creating a discovery protocol that has the desired security
properties may result in a design that is not efficient. To perform
the necessary operations the protocol may need to send and receive a
large number of network packets. This may consume an unreasonable
amount of network capacity (particularly problematic when it's shared
wireless spectrum), cause an unnecessary level of power consumption
(particularly problematic on battery devices) and may result in the
discovery process being slow.
It is a difficult challenge to design a discovery protocol that has
the property of obscuring the details of what it is doing from
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unauthorized observers, while also managing to do that quickly and
efficiently.
5.3. Secure Initialization and Trust Models
One of the challenges implicit in the preceding discussions is that
whenever we discuss "trusted entities" versus "untrusted entities",
there needs to be some way that trust is initially established, to
convert an "untrusted entity" into a "trusted entity".
One way to establish trust between two entities is to trust a third
party to make that determination for us. For example, the X.509
certificates used by TLS and HTTPS web browsing are based on the
model of trusting a third party to tell us who to trust. There are
some difficulties in using this model for establishing trust for
service discovery uses. If we want to print our tax returns or
medical documents on "our" printer, then we need to know which
printer on the network we can trust be be "our" printer. All of the
printers we discover on the network may be legitimate printers made
by legitimate printer manufacturers, but not all of them are "our"
printer. A third-party certificate authority cannot tell us which
one of the printers is ours.
Another common way to establish a trust relationship is Trust On
First Use (TOFU), as used by ssh. The first usage is a Leap Of
Faith, but after that public keys are exchanged and at least we can
confirm that subsequent communications are with the same entity. In
today's world, where there may be attackers present even at that
first use, it would be preferable to be able to establish a trust
relationship without requiring an initial Leap Of Faith.
Techniques now exist for securely establishing a trust relationship
without requiring an initial Leap Of Faith. Trust can be established
securely using a short passphrase or PIN with cryptographic
algorithms such as Secure Remote Password (SRP) [RFC5054] or a
Password Authenticated Key Exchange like J-PAKE [RFC8236] using a
Schnorr Non-interactive Zero-Knowledge Proof [RFC8235].
Such techniques require a user to enter the correct passphrase or PIN
in order for the cryptographic algorithms to establish working
communication. This avoids the human tendency to simply press the
"OK" button when asked if they want to do something on their
electronic device. It removes the human fallibility element from the
equation, and avoids the human users inadvertently sabotaging their
own security.
Using these techniques, if a user tries to print their tax return on
a printer they've never used before (even though the name looks
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right) they'll be prompted to enter a pairing PIN, and the user
*cannot* ignore that warning. They can't just press an "OK" button.
They have to walk to the printer and read the displayed PIN and enter
it. And if the intended printer is not displaying a pairing PIN, or
is displaying a different pairing PIN, that means the user may be
being spoofed, and the connection will not succeed, and the failure
will not reveal any secret information to the attacker. As much as
the human desires to "just give me an OK button to make it print"
(and the attacker desires them to click that OK button too) the
cryptographic algorithms do not give the user the ability to opt out
of the security, and consequently do not give the attacker any way to
persuade the user to opt out of the security protections.
5.4. External Dependencies
Trust establishment may depend on external, and optionally online,
parties. Systems which have such a dependency may be attacked by
interfering with communication to external dependencies. Where
possible, such dependencies should be minimized. Local trust models
are best for secure initialization in the presence of active
attackers.
6. IANA Considerations
This draft does not require any IANA action.
7. Acknowledgments
This draft incorporates many contributions from Stuart Cheshire and
Chris Wood.
8. Informative References
[K17] Kaiser, D., "Efficient Privacy-Preserving
Configurationless Service Discovery Supporting Multi-Link
Networks", 2017,
<http://nbn-resolving.de/urn:nbn:de:bsz:352-0-422757>.
[KW14a] Kaiser, D. and M. Waldvogel, "Adding Privacy to Multicast
DNS Service Discovery", DOI 10.1109/TrustCom.2014.107,
2014, <http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=7011331>.
[KW14b] Kaiser, D. and M. Waldvogel, "Efficient Privacy Preserving
Multicast DNS Service Discovery",
DOI 10.1109/HPCC.2014.141, 2014,
<http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=7056899>.
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